In recent years, a variety of components have been copolymerized, or polymers have been produced as wholly aromatic polymers such as polyarylate, for improving heat resistance of polyesters. A polyester from 9,9-bis(4-hydroxyphenyl)fluorene has been also proposed, while no proposal has been made for a copolyester from 9,9-bis[4-(2-hydroxyethoxy)phenyl]fluorene.
That is, U.S. Pat. No. 3,546,165 discloses that a polymer from 9,9-bis(4-hydroxyphenyl)fluorene, terephthalic acid and isophthalic acid is obtained by an acid chloride method thereof. For improving this polymer in moldability, Japanese Patent Publication No. 22,931/1992 discloses a heat-resistant polyester from a mixture prepared by mixing 9,9-bis(4-hydroxyphenyl)fluorene with terephthalic acid, isophthalic acid and a fatty acid in a specific mixing ratio. Further, Japanese Laid-open Patent Publication No. 152,622/1988 discloses a process for producing a soluble, heat-resistant polyarylate, and Japanese Laid-open Patent Publications Nos. 192432/1982 and 292830/1987 disclose raw materials having a specific viscosity for copolymers from mixtures of 9,9-bis(4-hydroxyphenyl)fluorene with terephthalic acid and isophthalic acid in specific mixing ratios. Further, a polyester from a mixture of this 9,9-bis(4-hydroxyphenyl)fluorene with terephthalic acid, isophthalic acid and an aliphatic dicarboxylic acid is also disclosed.
However, for all the above polymers, 9,9-bis(4-hydroxyphenyl)fluorene is used as one of raw materials, and the polymerization method therefor requires special polymerization conditions. That is, differing from an aliphatic alcohol, 9,9-bis(4-hydroxyphenyl)fluorene is difficult to react due to a phenolic hydroxyl group on each terminal thereof. Hence, in the melt polymerization method, reaction conditions at higher temperatures are required, and a polymer formed is likely to undergo thermal decomposition to cause coloring and degradation in quality. Further, it is generally said that, in a polymerization method in which a dicarboxylic acid is converted to an acid chloride and then hydrochloric acid is removed in a solvent, it is difficult to proceed with a uniform reaction, and that a polymer having a large molecular weight distribution is obtained. It is also required to post-treat a compound of a catalyst which is required in a large amount. Thus, the production method is complicated, and additional costs are required. Further, in spite of various improvements, the material obtained has too high a glass transition point, and injection-molding thereof is difficult. Therefore, it cannot be said that the moldability thereof has been satisfactory.
Transparent resins having excellent mechanical properties are conventionally used as an engineering plastic in the filed of optical materials. For example, polymethyl methacrylate (to be abbreviated as PMMA hereinafter), polycarbonate (to be abbreviated as PC hereinafter) and noncrystalline polyolefin and the like are used as optical materials for a compact disk, a laser disk and a lens or as materials for transparent parts of automobiles. However, the problems with these resins are that they have low heat resistance, that they show high moisture absorption and have poor stability in form, and that they have stress strain or molecular orientation in heat molding such as in injection molding to give a product having a high birefringence, or the materials therefor are very expensive. It therefore cannot be said that the above resins are fully satisfactory. In recent years, further, with prompt developments of optical disks using laser light to record and reproduce information such as voices, images and characters, it is desired to develop substrate materials having higher-performance optical properties.
The present inventors have made diligent studies in view of the above prior art defects and completed the present invention by finding a polyester having a specific structure. The object of the invention is to provide a copolyester which is excellent in transparency and mechanical and electrical properties and which has sufficient heat resistance in practical use.
It is another object of the present invention to provide a copolyester which has small optical anistropy, is excellent in moldability and dimensional stability and is suitable for an engineering plastic, particularly as an optical material.
It is further another object of the present invention to provide a copolyester which can be industrially-easily produced at a low cost; and a molded article therefrom.